Regulatory air pollution limits for diesel engines have caused some manufacturers to adopt engine aftertreatment systems (EASs) for treating diesel exhaust before it is released into the atmosphere. Various EAS functions include conversion of pollutant compounds in diesel exhaust into non-pollutant compounds and filtering of pollutant compounds from diesel exhaust.
Some EASs include selective catalytic reduction (SCR) technology for reducing nitrogen oxides (NOx) in diesel exhaust. The SCR process introduces a diesel exhaust fluid (DEF), such as an NOx reducing compound (e.g., a urea water solution), into the hot exhaust gas. The DEF chemically reduces pollutant compounds, such as NOx, into non-pollutant compounds in conjunction with a catalyst. Other EASs include diesel particulate filter (DPF) technology. The DPF filters soot from diesel exhaust, thereby reducing the amount of soot expelled from the vehicle. In some cases, the DPF also passively oxidizes diesel exhaust.
Some EASs have a combined SCR unit and DPF unit, sometimes referred to as an “SCRF.” The use of an SCRF in place of separate SCR and DPF units decreases cost while reducing size of the EAS. However, many EASs that include an SCRF unit also require an additional SCR unit or two to obtain the desired aftertreatment effect. The result of adding one or more additional SCR units is increased cost and overall size, as well as other disadvantages, such as increased pressure differential through the EAS and reduced fuel economy. A higher pressure differential through the EAS causes frequent active regenerations by control systems, which is undesirable for the stability of a catalyst in an SCR and can result in rapid hydrothermal catalytic aging.
With the combined efforts to decrease EAS size and increase EAS efficiency, an EAS is needed that permits the use of an SCRF without requiring the use of an additional SCR unit.